Brønsted acid sites based on penta-coordinated aluminum species.

Zichun Wang,Julien Trébosc,Olivier Lafon,Jun Huang,Jean-Paul Amoureux,Kyung Duk Kim,Catherine Stampfl,Yijiao Jiang,Alfons Baiker

doi:10.1038/ncomms13820

Abstract

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. It is widely believed that BASs consist only of tetra-coordinated aluminum sites (AlIV) with bridging OH groups in zeolites or nearby silanols on ASA surfaces. Here we report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning. Both BAS-AlIV and -AlV show a similar acidity to protonate probe molecular ammonia. The quantitative evaluation of 1H and 27Al sites demonstrates that BAS-AlV co-exists with BAS-AlIV rather than replaces it, which opens new avenues for strongly enhancing the acidity of these popular solid acids.

Highlights

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry
The proximity between AlIV and silanol sites in ASA has recently been observed by nuclear magnetic resonance (NMR) correlation experiments between 29Si and 27Al nuclei, the sensitivity of which was enhanced by dynamic nuclear polarization[22]
For ASA containing solely BASs based on AlIV species (BAS-AlIV), the maximum Brønsted acidity has been obtained at 30 wt% Al loading[37,38]

Summary

Introduction

Zeolites and amorphous silica-alumina (ASA), which both provide Brønsted acid sites (BASs), are the most extensively used solid acid catalysts in the chemical industry. We report the direct observation in ASA of a new type of BAS based on penta-coordinated aluminum species (AlV) by 27Al-{1H} dipolar-mediated correlation two-dimensional NMR experiments at high magnetic field under magic-angle spinning.

Results

Conclusion